Let Suppose 3 digit number as 3 boxes as shown below. First Box is at 100th place, the Second box is at 10th place, and the Third box be at 1st place.
To make a3 digit number,
1st box can be filled with nine numbers(1, 2, 3, 4, 5, 6, 7, 8, 9) if we include 0 in 1st box then it become 2 digit number(i.e 010 is 2 digit number not 3 digit)
2nd box can be filled with ten numbers(1, 2, 3, 4, 5, 6, 7, 8, 9, 0) as repetition is allowed.
Similarly 3rd box can be filled with ten numbers(1, 2, 3, 4, 5, 6, 7, 8, 9, 0)
Total number of ways is 9 x 10 x 10 = 900
